Conventional vortex shedding flow meters have vortex shedding generators that produce vortices alternating sequentially from each side of the generator producing two rows of vortices having opposing direction of rotation, in a formation known as a von Karman vortex street. The shedding frequency of these vortices is typically detected and measured by sensing the influence of a differential pressure between a fully formed vortex and a depleted vortex to determine the mean flow velocity. The frequency is linearly proportional to the mean flow velocity.
Sensors used to detect the vortices often include thin diaphragms that respond to the alternating differential pressure variations generated by the vortices. For example in U.S. Pat. No. 4,085,614 to Sgourakes et al., differential pressure is applied to the diaphragms and transferred to a piezoelectric element sealed within a sensor housing via an electrically non-conductive hydraulic fill fluid. This type of sensor cannot be used for measuring the flow velocity of process fluids at extreme temperatures mainly due to the operating temperature limitations of fill fluids and conventional piezoelectric elements used in these types of sensors. Meters used for measuring high temperature fluids are therefore traditionally constructed without fill fluids.
Such meters for measuring high temperature fluids without fill fluids may detect vortices with sensors located either within or external to the flow conduit. An example of meters with sensors within the flow conduit is shown in U.S. Pat. No. 5,003,827 to Kalinoski et al in which the vortices are sensed by a spool member slideably disposed in a chamber of the shedder that shuttles side to side as a result of the alternating differential pressures of the vortices. The spool member in turn generates mechanical forces upon the piezoelectric sensor. In this type of system, the piezoelectric sensor cannot be replaced without flow interruption, the process flow line must be vented, requires process seals and the piezoelectric sensor material must be conditioned and protected to provide a useful lifetime for operation at these high temperatures. An example of means to protect the piezoelectric sensor material with an oxygen diffusion path is shown in U.S. Pat. No. 7,650,798 Foster et al.
One example of meters for measuring high temperature fluid flow, with flow sensors external to the flow conduit, is shown in U.S. Pat. No. 4,891,990 to Khalifa. In this meter a flexible portion of the vortex generator bends a flat plate and piezoelectric sensors sense the bending of the plate. Other examples of vortex flow meters with the sensor external to the flow path exist are shown in U.S. Pat. No. 6,973,841 to Foster, which discloses a differential pressure responsive paddle as a part of the vortex shedding generator; the paddle applying a rocking motion to a region of reduced thickness in a portion of the conduit. The motion of the paddle is transmitted by a lever of relatively low stiffness compared to the piezoelectric sensor which results in a low force applied to the piezoelectric sensor thereby reducing the performance characteristics of the sensor. Piezoelectric sensors in this configuration are field replaceable with factory support. This design is further flawed, however, in that if the reduced thickness portion of the conduit fails to contain high process pressures and/or hazardous fluids.
These piezoelectric sensors operating at high temperatures are subject to reliability concerns as disclosed in U.S. Pat. No. 5,209,125 to Kalinoski et al, as well as U.S. Pat. No. 7,650,798. Both references disclose methods for improving reliability of piezoelectric sensors by conditioning the piezoelectric sensor operating in high temperature environments.
Thus present vortex flow meters with either the sensor located within the flow conduit or external to the flow conduit have concerns that are not addressed. A need exists for the elimination of these concerns to provide a user reliable, high performance and economical product.
Furthermore, most of the existing vortex flow meters do not provide a redundant measurement option in a wafer form for the relatively large components cannot be located within the bolt pattern required for wafer configuration.
Many flow meters with a wafer form are limited to low pressure four bolt flanges. The present alternative for high pressure flows are flanged flow meters. Redundant sensing requires costly-flanged dual meters or a much more costly-flanged dual in-line meter. The dual in-line meters require two flow K-factors and have increased pressure drop causing greater pumping losses.